1. Field of the Invention
The present invention relates to an exchange coupling film which is composed of a seed layer, an antiferromagnetic layer, and a ferromagnetic layer in that order from the bottom, wherein the direction of magnetization of the aforementioned ferromagnetic layer is set in a specified direction due to an exchange coupling magnetic field generated at the interface between the aforementioned antiferromagnetic layer and the ferromagnetic layer, and to a magnetic detection element (spin valve type thin film element, AMR element, etc.) using the aforementioned exchange coupling film. In particular, the present invention relates to an exchange coupling film capable of properly improving reliability in current-carrying (electromigration resistance) and of achieving excellent rate of resistance change, etc., compared to those heretofore attained even in the future increase in recording density, and to a magnetic detection element using the aforementioned exchange coupling film.
2. Description of the Related Art
FIG. 25 is a partial sectional view of a conventional magnetic detection element (spin valve type thin film element) cut from the direction parallel to the surface facing a recording medium.
Reference numeral 14 shown in FIG. 25 denotes a seed layer formed from, for example, NiFeCr, and an antiferromagnetic layer 30, a fixed magnetic layer 31, a non-magnetic material layer 32, a free magnetic layer 33, and a protection layer 7 are laminated in that order on the aforementioned seed layer 14.
Regarding this sort of spin valve type thin film element, an exchange coupling magnetic field is generated at the interface between the aforementioned antiferromagnetic layer 30 and fixed magnetic layer 31 due to a heat treatment and, therefore, magnetization of the aforementioned fixed magnetic layer 31 is fixed in the direction of height (the Y direction in the drawing).
In FIG. 25, hard bias layers 5 are formed on both sides of the multilayer film from the aforementioned seed layer 14 to the protection layer 7, and by a vertical bias magnetic field from the aforementioned hard bias layers 5, magnetization of the aforementioned free magnetic layer 33 is arranged in the direction of track width (the X direction in the drawing).
As shown in FIG. 25, electrode layers 8 are formed while being overlaid on the aforementioned hard bias layers 5. A sense current from the electrode layers 8 primarily passes through three layers of the fixed magnetic layer 31, the non-magnetic material layer 32, and the free magnetic layer 33.
In the spin valve type thin film element shown in FIG. 25, the seed layer 14 has been formed under the aforementioned antiferromagnetic layer 30, and improvements of reliability in current-carrying represented by an improvement of electromigration resistance and rate of resistance change have been expected.
Hitherto, it was considered important that the crystal structure of the aforementioned seed layer 14 was a face-centered cubic structure (fcc structure).
When the aforementioned seed layer 14 had the face-centered cubic structure, each layer formed thereon was able to properly bring about {111} orientation, and the crystal particle diameter was able to increase. Consequently, scattering of conduction electrons at the grain boundaries was reduced, the electrical conductivity was improved and, in addition, an exchange coupling magnetic field which was generated at the interface between the fixed magnetic layer 30 and antiferromagnetic layer 31 was increased, and improvements of he reliability in current-carrying and the like were expected.
Hitherto, the aforementioned seed layer 14 was formed from a NiFeCr alloy, at that time, the compositional ratio of the aforementioned Cr was set at 40 at % or less and, thereby, the crystal structure of the aforementioned seed layer 14 was maintained to be a face-centered cubic structure.
However, accompanying the future increase in recording density, since the spin valve type thin film element is further miniaturized, the density of the sense current, which passes through the aforementioned spin valve type thin film element, is increased. Therefore, generation of electromigration and the like have become problems.
Consequently, it has been required to improve properly the material to become the seed layer 14, and to develop a seed layer capable of exhibiting further excellent characteristics compared to those of the seed layer 14 formed from the NiFeCr alloy having the compositional ratio of Cr of 40 at % or less.
Herein, when the inventors of the present invention formed the aforementioned seed layer 14 from a single layer of Cr, the aforementioned exchange coupling magnetic field larger than that in the case where the aforementioned seed layer was formed from the NiFeCr alloy (Cr was 40 at %) was achieved and, therefore, an improvement of the reliability in current-carrying characteristic represented by the electromigration resistance was improved compared to those which had been attained up to that time.
On the other hand, the rate of resistance change (ΔR/R) of the seed layer 14 formed from Cr tended to become smaller than that of the seed layer formed from the NiFeCr alloy having the compositional ratio of Cr of 40 at % or less. In particular, that was observed more remarkably with an increase in the film thickness of Cr. Consequently, regarding the seed layer formed from Cr, it was difficult to improve the reliability in current-carrying and the rate of resistance change simultaneously.
When the conventional NiFeCr alloy having the compositional ratio of Cr of 40 at % or less was used as the seed layer 14, waves among crystal particles were generated on the surface of the antiferromagnetic layer 30, the smoothness of the surface of the aforementioned antiferromagnetic layer 30 was degraded and, thereby, the following problems were brought about.
FIG. 26 is a partial schematic diagram of the structure of the magnetic detection element shown in FIG. 25 under magnification. As shown in FIG. 26, it is clear that waves are generated on the surface 30a among crystal particles formed on the aforementioned antiferromagnetic layer 30. These waves are also generated on the surfaces of the fixed magnetic layer 31, the non-magnetic material layer 32, and the free magnetic layer 33 formed on the aforementioned antiferromagnetic layer 30.
When these waves are generated, as shown in FIG. 27 (a schematic diagram showing the cross section of the fixed magnetic layer 31, the non-magnetic material layer 32, and the free magnetic layer 33 shown in FIG. 26 cut in the Y direction), magnetic poles are generated at the wave portions on the fixed magnetic layer 31 surface, the aforementioned magnetic poles are also generated at the wave portions of the free magnetic layer 33 which faces interposing the non-magnetic material layer 32 and, thereby, a ferromagnetic coupling magnetic field Hin due to a magnetostatic coupling (topological coupling) between the fixed magnetic layer 31 and the free magnetic layer 33 is strengthened. Therefore, an action that tends to magnetize the free magnetic layer 33, which must be magnetized essentially in the X direction shown in the drawing, in the Y direction shown in the drawing is effected. Consequently, problems have occurred in that asymmetry of the playback waveform is increased and the like.
A mirror reflection layer formed from, for example, an oxide of Ta, may be formed on the aforementioned free magnetic layer 33. In such a case, the smoothness of the surface of the aforementioned mirror reflection layer has also been Hindered by the waves on the surface 30a of the antiferromagnetic layer 30 and, thereby, the mirror reflectivity of the aforementioned mirror reflection layer has been reduced, and it has not been possible to expect increase of the rate of resistance change due to a specular effect.